Transducers such as ultrasonic transducers are provided in a wide variety of electronic applications. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase as well. This has lead to comparatively small transducers, which may be micromachined according to technologies such as micro-electromechanical systems (MEMS) technology. One type of transducer is a piezoelectric micromachined ultrasonic transducer (PMUT). The PMUT includes a layer of piezoelectric material between two conductive plates (electrodes) thereby forming a membrane. When functioning as a receiver, an acoustic wave incident on the membrane results in the application of a time varying force to the piezoelectric material. Application of the time-varying force to a piezoelectric material results in induced stresses in the piezoelectric material, which in-turn creates a time-varying voltage signal across the material. This time-varying voltage signal may be measured by sensor circuits to determine the characteristics of the incident acoustic wave. Alternatively, this time-varying voltage signal may produce a time-varying charge that is provided to sensor circuits that process the signal and determine the characteristics of the incident acoustic wave. When functioning as a transmitter, a voltage excitation produces vibration of the diaphragm. This in turn radiates acoustic energy into the air (or any gaseous medium).
The layers that comprise the membrane of active area of PMUTs are typically comparatively thin. As can be appreciated, PMUTs are fragile and require delicate handling during processing. For example, after a plurality of PMUTs are fabricated over a wafer, individual PMUTs or groups of PMUTs are separated from one another in a process known as singulation. However, because of the comparatively fragile membrane of the PMUT, many known integrated circuit dicing techniques, such as wafer sawing, are difficult or impossible to use without damaging the fragile membrane.
A common technique used to singulate PMUTs is the scribe-and-break process. However, the process has several drawbacks. A successful scribe-and-break process requires a large aspect ratio of the die length to wafer thickness. This allows enough leverage to be generated to result in a clean break of the wafer. However, such a large die results in a high die cost. An alternative is to the so-called scribe-and-break process involves thinning the wafer comprising the plurality of PMUTs, thus reducing the aspect ratio. Unfortunately, in many PMUT devices a comparatively large cavity is needed to provide desired acoustic properties of the PMUT. As can be appreciated, not only is the thinned die fragile, the desired comparatively deep cavity is reduced by the wafer thinning. Therefore significant wafer thinning is likely to degrade the performance of such devices.
There is a need, therefore, for a method of fabricating a transducer that addresses at least the shortcomings described above.